Is there any way to hook up a big rig to a trailer? There's a place in the first main island where a truck is parked near some trailers where you can get the back end to go under them but it won't hook up. Unless they took it out of IV, which i hope isnt the case.

I posted a topic with the same question... I guess no, got no reply. I actually finished the game and there is no mission where you would learn to (as I kept hoping...). I hope they will add this in the DLC or a patch... it was awesome.

Rockstar probably removed trailers because of the fact that IV was just a single city. There was no need to have such vehicles in a city with such small streets.
A big rig and trailer just wouldn't fit in.